Virtually imaged phased array (VIPA) having air between reflecting surfaces

ABSTRACT

An apparatus which can be referred to as a Virtually Imaged Phased Array (VIPA). The apparatus receives an input light and produces a spatially distinguishable output light in accordance with the wavelength of the input light. The apparatus includes first and second surfaces separated from each other with air in between. The second surface has a reflectivity which allows a portion of light incident thereon to be transmitted therethrough. The first and second surfaces are positioned so that the input light is reflected a plurality of times between the first and second surfaces through the air to cause a plurality of lights to be transmitted through the second surface. The plurality of transmitted lights interfere with each other to produce the output light. Moreover, the input light is at a respective wavelength within a continuous range of wavelengths, and the output light is spatially distinguishable from an output light formed for an input light having any other wavelength within the continuous range of wavelengths.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 08/796,842,filed Feb. 7, 1997, now pending, which is a continuation-in-part ofapplication Ser. No. 08/685,362, filed Jul. 24, 1996, now pending.

This application claims priority to Japanese patent application Ser. No.07-190535, filed Jul. 26, 1995, in Japan, and which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus producing chromaticdispersion, and which can be used to compensate for chromatic dispersionaccumulated in an optical fiber transmission line. More specifically,the present invention relates to an apparatus which uses a virtuallyimaged phased array to produce chromatic dispersion.

2. Description of the Related Art

FIG. 1(A) is a diagram illustrating a conventional fiber opticcommunication system, for transmitting information via light. Referringnow to FIG. 1(A), a transmitter 30 transmits pulses 32 through anoptical fiber 34 to a receiver 36. Unfortunately, chromatic dispersion,also referred to as "wavelength dispersion", of optical fiber 34degrades the signal quality of the system.

More specifically, as a result of chromatic dispersion, the propagatingspeed of a signal in an optical fiber depends on the wavelength of thesignal. For example, when a pulse with a longer wavelength (for example,a pulse with wavelengths representing a "red" color pulse) travelsfaster than a pulse with a shorter wavelength (for example, a pulse withwavelengths representing a "blue" color pulse), the dispersion istypically referred to as "normal" dispersion. By contrast, when a pulsewith a shorter wavelength (such as a blue color pulse) is faster than apulse with a longer wavelength (such as a red color pulse), thedispersion is typically referred to as "anomalous" dispersion.

Therefore, if pulse 32 consists of red and blue color pulses is whenemitted from transmitter 30, pulse 32 will be split as it travelsthrough optical fiber 34 so that a separate red color pulse 38 and ablue color pulse 40 are received by receiver 36 at different times. FIG.1(A) illustrates a case of "normal" dispersion, where a red color pulsetravels faster than a blue color pulse.

As another example of pulse transmission, FIG. 1(B) is a diagramillustrating a pulse 42 having wavelength components continuously fromblue to red, and transmitted by transmitter 30. FIG. 1(C) is a diagramillustrating pulse 42 when arrived at receiver 36. Since the redcomponent and the blue component travel at different speeds, pulse 42 isbroadened in optical fiber 34 and, as illustrated by FIG. 1(C), isdistorted by chromatic dispersion. Such chromatic dispersion is verycommon in fiber optic communication systems, since all pulses include afinite range of wavelengths.

Therefore, for a fiber optic communication system to provide a hightransmission capacity, the fiber optic communication system mustcompensate for chromatic dispersion.

FIG. 2 is a diagram illustrating a fiber optic communication systemhaving an opposite dispersion component to compensate for chromaticdispersion. Referring now to FIG. 2, generally, an opposite dispersioncomponent 44 adds an "opposite" dispersion to a pulse to canceldispersion caused by travelling through optical fiber 34.

There are conventional devices which can be used as opposite dispersioncomponent 44. For example, FIG. 3 is a diagram illustrating a fiberoptic communication system having a dispersion compensation fiber whichhas a special cross-section index profile and thereby acts as anopposite dispersion component, to compensate for chromatic dispersion.Referring now to FIG. 3, a dispersion compensation fiber 46 provides anopposite dispersion to cancel dispersion caused by optical fiber 34.However, a dispersion compensation fiber is expensive to manufacture,and must be relatively long to sufficiently compensate for chromaticdispersion. For example, if optical fiber 34 is 100 km in length, thendispersion compensation fiber 46 should be approximately 20 to 30 km inlength.

FIG. 4 is a diagram illustrating a chirped grating for use as anopposite dispersion component, to compensate for chromatic dispersion.Referring now to FIG. 4, light travelling through an optical fiber andexperiencing chromatic dispersion is provided to an input port 48 of anoptical circulator 50. Circulator 50 provides the light to chirpedgrating 52. Chirped grating 52 reflects the light back towardscirculator 50, with different wavelength components reflected atdifferent distances along chirped grating 52 so that differentwavelength components travel different distances to thereby compensatefor chromatic dispersion. For example, chirped grating 52 can bedesigned so that longer wavelength components are reflected at a fartherdistance along chirped grating 52, and thereby travel a farther distancethan shorter wavelength components. Circulator 50 then provides thelight reflected from chirped grating 52 to an output port 54. Therefore,chirped grating 52 can add opposite dispersion to a pulse.

Unfortunately, a chirped grating has a very narrow bandwidth forreflecting pulses, and therefore cannot provide a wavelength bandsufficient to compensate for light including many wavelengths, such as awavelength division multiplexed light. A number of chirped gratings maybe cascaded for wavelength multiplexed signals, but this results in anexpensive system. Instead, a chirped grating with a circulator, as inFIG. 4, is more suitable for use when a single channel is transmittedthrough a fiber optic communication system.

FIG. 5 is a diagram illustrating a conventional diffraction grating,which can be used in producing chromatic dispersion. Referring now toFIG. 5, a diffraction grating 56 has a grating surface 58. Parallellights 60 having different wavelengths are incident on grating surface58. Lights are reflected at each step of grating surface 58 andinterfere with each other. As a result, lights 62, 64 and 66 havingdifferent wavelengths are output from diffraction grating 56 atdifferent angles. A diffraction grating can be used in a spatial gratingpair arrangement, as discussed in more detail below, to compensate forchromatic dispersion.

More specifically, FIG. 6(A) is a diagram illustrating a spatial gratingpair arrangement for use as an opposite dispersion component, tocompensate for chromatic dispersion. Referring now to FIG. 6 (A), light67 is diffracted from a first diffraction grating 68 into a light 69 forshorter wavelength and a light 70 for longer wavelength. These lights 69and 70 are then diffracted by a second diffraction grating 71 intolights propagating in the same direction. As can be seen from FIG. 6(A),wavelength components having different wavelengths travel differentdistances, to add opposite dispersion and thereby compensate forchromatic dispersion. Since longer wavelengths (such as lights 70)travel longer distance than shorter wavelengths (such as lights 69), aspatial grating pair arrangement as illustrated in FIG. 6(A) hasanomalous dispersion.

FIG. 6(B) is a diagram illustrating an additional spatial grating pairarrangement for use as an opposite dispersion component, to compensatefor chromatic dispersion. As illustrated in FIG. 6(B), lenses 72 and 74are positioned between first and second diffraction gratings 68 and 71so that they share one of the focal points. Since longer wavelengths(such as lights 70) travel shorter distance than shorter wavelengths(such as lights 69), a spatial grating pair arrangement as illustratedin FIG. 6(B) has normal dispersion.

A spatial grating pair arrangement as illustrated in FIGS. 6(A) and 6(B)is typically used to control dispersion in a laser resonator. However, apractical spatial grating pair arrangement cannot provide a large enoughdispersion to compensate for the relatively large amount of chromaticdispersion occurring in a fiber optic communication system. Morespecifically, the angular dispersion produced by a diffraction gratingis usually extremely small, and is typically approximately 0.05degrees/nm. Therefore, to compensate for chromatic dispersion occurringin a fiber optic communication system, first and second gratings 68 and71 would have to be separated by very large distances, thereby makingsuch a spatial grating pair arrangement impractical.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide anapparatus which produces chromatic dispersion, and which is practicalfor compensating for chromatic dispersion accumulated in an opticalfiber.

Objects of the present invention are achieved by providing an apparatuswhich includes a device herein referred to as a "virtually imaged phasedarray", or "VIPA". The VIPA produces a light propagating away from theVIPA. The apparatus also includes a light returning device which returnsthe light back to the VIPA to undergo multiple reflection inside theVIPA. The light returning device can be arranged to return light back tothe VIPA having a respective interference order, without returning lightback to the VIPA having any other interference order.

Objects of the present invention are also achieved by providing anapparatus which includes a VIPA that receives an input light having awavelength within a continuous range of wavelengths and produces acontinuously corresponding output light. The output light is spatiallydistinguishable (for example, it travels in a different direction) froman output light formed for an input light having any other wavelengthwithin the continuous range of wavelengths. If the output light isdistinguishable by its travelling angle, the apparatus has an angulardispersion.

Further, objects of the present invention are achieved by providing aVIPA and a light returning device, wherein the VIPA includes a passagearea and a transparent material. The passage area allows light to bereceived into, and be output from, the VIPA. The transparent materialhas first and second surfaces thereon, the second surface having areflectivity which allows a portion of light incident thereon to betransmitted therethrough. An input light is received in the VIPA throughthe passage area and is reflected a plurality of times in thetransparent material between the first and second surfaces to cause aplurality of lights to be transmitted through the second surface. Theplurality of transmitted lights interfere with each other to produce anoutput light. The input light is at a wavelength within a continuousrange of wavelengths and the output light is spatially distinguishablefrom an output light formed for an input light having any otherwavelength within the continuous range of wavelengths. The lightreturning device causes the output light to be returned in the exactlyopposite direction back to the second surface and pass therethrough intothe VIPA so that the output light undergoes multiple reflection in theVIPA and is then output from the passage area of the VIPA to the inputpath.

In addition, object of the present invention are achieved by providingan apparatus which includes a VIPA that produces a plurality of outputlights at the same wavelength of the input light and having differentinterference orders. The apparatus also includes a light returningdevice which returns the output light in one of the interference ordersto the VIPA, and does not return the other output lights. In thismanner, only light corresponding to a single interference order isreturned back to the VIPA.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe preferred embodiments, taken in conjunction with the accompanyingdrawings of which:

FIG. 1(A) (prior art) is a diagram illustrating a conventional fiberoptic communication system.

FIG. 1(B) is a diagram illustrating a pulse before transmission througha fiber in a conventional fiber optic communication system.

FIG. 1(C) is a diagram illustrating a pulse after being transmittedthrough a fiber in a conventional fiber optic communication system.

FIG. 2 (prior art) is a diagram illustrating a fiber optic communicationsystem having an opposite dispersion component to compensate forchromatic dispersion.

FIG. 3 (prior art) is a diagram illustrating a fiber optic communicationsystem having a dispersion compensation fiber as an opposite dispersioncomponent.

FIG. 4 (prior art) is a diagram illustrating a chirped grating for useas an opposite dispersion component, to compensate for chromaticdispersion.

FIG. 5 (prior art) is a diagram illustrating a conventional diffractiongrating.

FIG. 6(A) (prior art) is a diagram illustrating a spatial grating pairarrangement for production of anomalous dispersion.

FIG. 6(B) (prior art) is a diagram illustrating a spatial grating pairarrangement for production of normal dispersion.

FIG. 7 is a diagram illustrating a VIPA, according to an embodiment ofthe present invention.

FIG. 8 is a detailed diagram illustrating the VIPA of FIG. 7, accordingto an embodiment of the present invention.

FIG. 9 is a diagram illustrating a cross-section along lines IX--IX ofthe VIPA illustrated in FIG. 7, according to embodiment of the presentinvention.

FIG. 10 is a diagram illustrating interference between reflectionsproduced by a VIPA, according to an embodiment of the present invention.

FIG. 11 is a diagram illustrating a cross-section along lines IX--IX ofthe VIPA illustrated in FIG. 7, for determining the tilt angle of inputlight, according to an embodiment of the present invention.

FIGS. 12(A), 12(B), 12(C) and 12(D) are diagrams illustrating a methodfor producing a VIPA, according to an embodiment of the presentinvention.

FIG. 13 is a diagram illustrating an apparatus which uses a VIPA as anangular dispersion component to produce chromatic dispersion, accordingto an embodiment of the present invention.

FIG. 14 is a more detailed diagram illustrating the operation of theapparatus in FIG. 13, according to an embodiment of the presentinvention.

FIG. 15 is a diagram illustrating various orders of interference of aVIPA, according to an embodiment of the present invention.

FIG. 16 is a graph illustrating the chromatic dispersion for severalchannels of a wavelength division multiplexed light, according to anembodiment of the present invention.

FIG. 17 is a diagram illustrating different channels of a wavelengthdivision multiplexed light being focused at different points on a mirrorby a VIPA, according to an embodiment of the present invention.

FIG. 18 is a diagram illustrating a side view of an apparatus which usesa VIPA to provide variable chromatic dispersion to light, according toan embodiment of the present invention.

FIG. 19 is a diagram illustrating a side view of an apparatus which usesa VIPA to provide variable chromatic dispersion to light, according toan additional embodiment of the present invention.

FIGS. 20(A) and 20(B) are diagrams illustrating side views of anapparatus which uses a VIPA to provide chromatic dispersion to light,according to additional embodiments of the present invention.

FIG. 21 is a diagram illustrating a side view of an apparatus which usesa VIPA to provide variable dispersion to light, according to a furtherembodiment of the present invention.

FIG. 22 is a diagram illustrating a top view of the apparatus in FIG.13, combined with a circulator, according to an embodiment of thepresent invention.

FIG. 23 is a diagram illustrating a top view of an apparatus using aVIPA, according to an embodiment of the present invention.

FIG. 24 is a diagram illustrating a controller for controlling thetemperature of a VIPA, according to an embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings, wherein like reference numerals refer tolike elements throughout.

FIG. 7 is a diagram illustrating a virtually imaged phased array (VIPA),according to an embodiment of the present invention. Moreover,hereinafter, the terms "virtually imaged phased array" and "VIPA" may beused interchangeably.

Referring now to FIG. 7, a VIPA 76 is preferably made of a thin plate ofglass. An input light 77 is focused into a line 78 with a lens 80, suchas a semi-cylindrical lens, so that input light 77 travels into VIPA 76.Line 78 is hereinafter referred to as "focal line 78". Input light 77radially propagates from focal line 78 to be received inside VIPA 76.VIPA 78 then outputs a luminous flux 82 of collimated light, where theoutput angle of luminous flux 82 varies as the wavelength of input light77 changes. For example, when input light 77 is at a wavelength λ1, VIPA76 outputs a luminous flux 82a at wavelength λ1 in a specific direction.When input light 77 is at a wavelength λ2, VIPA 76 outputs a luminousflux 82b at wavelength λ2 in a different direction. Therefore, VIPA 76produces luminous fluxes 82a and 82b which are spatially distinguishablefrom each other.

FIG. 8 is a detailed diagram illustrating VIPA 76, according to anembodiment of the present invention. Referring now to FIG. 8, VIPA 76includes a plate 120 made of, for example, glass, and having reflectingfilms 122 and 124 thereon. Reflecting film 122 preferably has areflectance of approximately 95% or higher, but less than 100%.Reflecting film 124 preferably has a reflectance of approximately 100%.A radiation window 126 is formed on plate 120 and preferably has areflectance of approximately 0% reflectance.

Input light 77 is focused into focal line 78 by lens 80 throughradiation window 126, to undergo multiple reflection between reflectingfilms 122 and 124. Focal line 78 is preferably on the surface of plate120 to which reflecting film 122 is applied. Thus, focal line 78 isessentially line focused onto reflecting film 122 through radiationwindow 126. The width of focal line 78 can be referred to as the "beamwaist" of input light 77 as focused by lens 80. Thus, the embodiment ofthe present invention as illustrated is FIG. 8 focuses the beam waist ofinput light 77 onto the far surface (that is, the surface havingreflecting film 122 thereon) of plate 120. By focusing the beam waist onthe far surface of plate 120, the present embodiment of the presentinvention reduces the possibility of overlap between (i) the area ofradiation window 126 on the surface of plate 120 covered by input light77 as it travels through radiation window 126 (for example, the area "a"illustrated in FIG. 11, discussed in more detail further below), and(ii) the area on reflecting film 124 covered by input light 77 wheninput light 77 is reflected for the first time by reflecting film 124(for example, the area "b" illustrated in FIG. 11, discussed in moredetail further below). It is desirable to reduce such overlap to ensureproper operation of the VIPA.

In FIG. 8, an optical axis 132 of input light 77 has a small tilt angleθ. Upon the first reflection off of reflecting film 122, 5% of the lightpasses through reflecting film 122 and diverges after the beam waist,and 95% of the light is reflected towards reflecting film 124. Afterbeing reflecting by reflecting film 124 for the first time, the lightagain hits reflecting film 122 but is displaced by an amount d. Then, 5%of the light passes through reflecting film 122. In a similar manner, asillustrated in FIG. 8, the light is split into many paths with aconstant separation d. The beam shape in each path forms so that thelight diverges from virtual images 134 of the beam waist. Virtual images134 are located with constant spacing 2t along a line that is normal toplate 120, where t is the thickness of plate 120. The positions of thebeam waists in virtual images 134 are self-aligned, and there is no needto adjust individual positions. The lights diverging from virtual images134 interfere with each other and form collimated light 136 whichpropagates in a direction that changes in accordance with the wavelengthof input light 77.

The spacing of light paths is d=2tSinθ, and the difference in the pathlengths between adjacent beams is 2tCosθ. The angular dispersion isproportional to the ratio of these two numbers, which is cotθ. As aresult, a VIPA produces a significantly large angular dispersion.

As easily seen from FIG. 8, the term "virtually imaged phased array"arises from the formation of an array of virtual images 134.

FIG. 9 is a diagram illustrating a cross-section along lines IX--IX ofVIPA 76 illustrated in FIG. 7, according to embodiment of the presentinvention. Referring now to FIG. 9, plate 120 has reflecting surfaces122 and 124 thereon. Reflecting surfaces 122 and 124 are in parallelwith each other and spaced by the thickness t of plate 120. Reflectingsurfaces 122 and 124 are typically reflecting films deposited on plate120. As previously described, reflecting surface 124 has a reflectanceof approximately 100%, except in radiation window 126, and reflectingsurface 122 has a reflectance of approximately 95% or higher. Therefore,reflecting surface 122 has a transmittance of approximately 5% or lessso that approximately 5% of less of light incident on reflecting surface122 will be transmitted therethrough and approximately 95% or more ofthe light will be reflected. The reflectances of reflecting surfaces 122and 124 can easily be changed in accordance with the specific VIPAapplication. However, generally, reflecting surface 133 should have areflectance which is less than 100% so that a portion of incident lightcan be transmitted therethrough.

Reflecting surface 124 has radiation window 126 thereon. Radiationwindow 126 allows light to pass therethrough, and preferably has noreflectance, or a very low reflectance. Radiation window 126 receivesinput light 77 to allow input light 77 to be received between, andreflected between, reflecting surfaces 122 and 124.

Since FIG. 9 represents a cross-section along lines IX--IX in FIG. 7,focal line 78 in FIG. 7 appears as a "point" in FIG. 9. Input light 77then propagates radially from focal line 78. Moreover, as illustrated inFIG. 9, focal line 78 is positioned on reflecting surface 122. Althoughit is not required for focal line 78 to be on reflecting surface 122, ashift in the positioning of focal line 78 may cause small changes in thecharacteristics of VIPA 76.

As illustrated in FIG. 9, input light 77 enters plate 120 through anarea A0 in radiation window 126, where points P0 indicate peripheralpoints of area A0.

Due to the reflectivity of reflecting surface 122, approximately 95% ormore of input light 77 is reflected by reflecting surface 122 and isincident on area Al of reflecting surface 124. Points P1 indicateperipheral points of area Al. After reflecting off area Al on reflectingsurface 124, input light 77 travels to reflecting surface 122 and ispartially transmitted through reflecting surface 122 as output lightOut1 defined by rays R1. In this manner, as illustrated in FIG. 9, inputlight 77 experiences multiple reflections between reflecting surfaces122 and 124, wherein each reflection off of reflecting surface 122 alsoresults in a respective output light being transmitted therethrough.Therefore, for example, input light 77 reflects off of areas A2, A3 andA4 to produce output lights Out2, Out3 and Out4. Points P2 indicateperipheral points of area A2, points P3 indicate peripheral points ofarea A3, and points P4 indicate peripheral points of area A4. Outputlight Out2 is defined by rays R2, output light Out3 is defined by raysR3 and output light Out4 is defined by rays R4. Although FIG. 9 onlyillustrates output lights Out0, Out1, Out2, Out3 and Out4, there willactually be many more output lights, depending on the power on inputlight 77 and the reflectances of reflecting surfaces 122 and 124. Aswill be discussed in more detail further below, the output lightsinterfere with each other to produce a luminous flux having a directionwhich changes in accordance with the wavelength of input light 77.

FIG. 10 is a diagram illustrating interference between reflectionsproduced by a VIPA, according to an embodiment of the present invention.Referring now to FIG. 10, light travelling from focal line 78 isreflected by reflecting surface 124. As previously described, reflectingsurface 124 has a reflectance of approximately 100% and, therefore,functions essentially as a mirror. As a result, output light Out1 can beoptically analyzed as if reflecting surfaces 122 and 124 did not existand, instead, output light Out1 was emitted from a focal line I₁.Similarly, output lights Out2, Out3 and Out4 can be optically analyzedas if they were emitted from focal lines I₁, I₂, and I₄, respectively.The focal lines I₂, I₃ and I₄ are virtual images of a focal line I₀.

Therefore, as illustrated in FIG. 10, focal line I₁ is a distance 2tfrom focal line I₀, where t equals the distance between reflectingsurfaces 122 and 124. Similarly, each subsequent focal line is adistance 2t from the immediately preceding focal line. Thus, focal lineI₂ is a distance 2t from focal line I₁. Moreover, each subsequentmultiple reflection between reflecting surfaces 122 and 124 produces anoutput light which is weaker in intensity than the previous outputlight. Therefore, output light Out2 is weaker in intensity than outputlight Out1.

As illustrated in FIG. 10, output lights from the focal lines overlapand interfere with each other. This interference produces a luminousflux which travels in a specific direction depending on the wavelengthof input light 77.

A VIPA according to the above embodiments of the present invention hasstrengthening conditions which are characteristics of the design of theVIPA. The strengthening conditions increase the interference of theoutput lights so that a luminous flux is formed. The strengtheningconditions of the VIPA are represented by the following Equation (1):

    2t×cosθ=mλ

where θ indicates the propagation direction of the resulting luminousflux as measured from a line perpendicular to the surface of reflectingsurfaces 122 and 124, λ indicates the wavelength of the input light, tindicates the distance between the reflecting surfaces 122 and 124, andm indicates an integer.

Therefore, if t is constant and m is assigned a specific value, then thepropagation direction θ of the luminous flux formed for input lighthaving wavelength λ can be determined.

More specifically, input light 77 is radially dispersed from focal line78 through a specific angle. Therefore, input light having the samewavelength will be travelling in many different direction from focalline 78, to be reflected between reflecting surfaces 122 and 124. Thestrengthening conditions of the VIPA cause light travelling in aspecific direction to be strengthened through interference of the outputlights to form a luminous flux having a direction corresponding to thewavelength of the input light. Light travelling in different directionthan the specific direction required by the strengthening condition willbe weakened by the interference of the output lights.

FIG. 11 is a diagram illustrating a cross-section along lines IX--IX ofthe VIPA illustrated in FIG. 7, showing characteristics of a VIPA fordetermining the angle of incidence, or tilt angle, of input light,according to an embodiment of the present invention.

Referring now to FIG. 11, input light 77 is collected by a cylindricallens (not illustrated) and focused at focal line 78. As illustrated inFIG. 11, input light 77 covers an area having a width equal to "a" onradiation window 126. After input light 77 is reflected one time fromreflecting surface 122, input light 77 is incident on reflecting surface124 and covers an area having a width equal to "b" on reflecting surface124. Moreover, as illustrated in FIG. 11, input light 77 travels alongoptical axis 132 which is at a tilt angle θ1 with respect to the normalto reflecting surface 122.

The tilt angle θ1 should be set to prevent input light 77 fromtravelling out of radiation window 126 after being reflected the firsttime by reflecting surface 122. In other words, the tilt angle θ1 shouldbe set- so that input light 77 remains "trapped" between reflectingsurfaces 122 and 124 and does not escape through radiation window 126.Therefore, to prevent input light 77 from travelling out of radiationwindow 126, the tilt angle θ1 should be set in accordance with thefollowing Equation (2):

    tilt of optical axis θ1 ≧(a+b)/4t

Therefore, as illustrated by FIGS. 7-11, embodiments of the presentinvention include a VIPA which receives an input light having arespective wavelength within a continuous range of wavelengths. The VIPAcauses multiple reflection of the input light to produceself-interference and thereby form an output light. The output light isspatially distinguishable from an output light formed for an input lighthaving any other wavelength within the continuous range of wavelengths.For example, FIG. 9 illustrates an input light 77 which experiencesmultiple reflection between reflecting surfaces 122 and 124. Thismultiple reflection produces a plurality of output lights Out0, Out1,Out2, Out3 and Out 4 which interfere with each other to produce aspatially distinguishable luminous flux for each wavelength of inputlight 77.

"Self-interference" is a term indicating that interference occursbetween a plurality of lights or beams which all originate from the samesource. Therefore, the interference of output lights Out0, Out1, Out2,Out3 and Out4 is referred to as self-interference of input light 77,since output lights Out0, Out1, Out2, Out3 and Out4 all originate fromthe same source (that is, input light 77).

According to the above embodiments of the present invention, an inputlight can be at any wavelength within a continuous range of wavelengths.Thus, the input light is not limited to being a wavelength which is avalue chosen from a range of discrete values.

In addition, according to the above embodiments of the presentinvention, the output light produced for an input light at a specificwavelength within a continuous range of wavelengths is spatiallydistinguishable from an output light which would have been produced ifthe input light was at a different wavelength within the continuousrange of wavelengths. Therefore, as illustrated, for example, in FIG. 7,the travelling direction (that is, a "spatial characteristic") of theluminous flux 82 is different when input light 77 is at differentwavelengths within a continuous range of wavelengths.

FIGS. 12(A), 12(B), 12(C) and 12(D) are diagram illustrating a methodfor producing a VIPA, according to an embodiment of the presentinvention.

Referring now to FIG. 12(A), a parallel plate 164 is preferably made ofglass and exhibits excellent parallelism. Reflecting films 166 and 168are formed on both sides of the parallel plate 164 by vacuum deposition,ion spattering or other such methods. One of reflecting films 166 and168 has a reflectance of nearly 100%, and the other reflecting film hasa reflectance of lower than 100%, and preferably higher than 80%.

Referring now to FIG. 12(B), one of reflecting films 166 and 168 ispartially shaved off to form a radiation window 170. In FIG. 12(B),reflecting film 166 is shown as being shaved off so that radiationwindow 170 can be formed on the same surface of parallel plate 164 asreflecting film 166. However, instead, reflecting film 168 can bepartially shaved off so that a radiation window is formed on the samesurface of parallel plate 164 as reflecting film 168. As illustrated bythe various embodiment of the present invention, a radiation window canbe on either side of parallel plate 164.

Shaving off a reflecting film can be performed by an etching process,but a mechanical shaving process can also be used and is less expensive.However, if a reflecting film is mechanically shaved, parallel plate 164should be carefully processed to minimize damage to parallel plate 164.For example, if the portion of parallel plate 164 forming the radiationwindow is severely damaged, parallel plate 164 will generate excess losscaused by scattering of received input light.

Instead of first forming a reflecting film and then shaving it off, aradiation window can be produced by preliminarily masking a portion ofparallel plate 164 corresponding to a radiation window, and thenprotecting this portion from being covered with reflecting film.

Referring now to FIG. 12 (C), a transparent adhesive 172 is applied ontoreflecting film 166 and the portion of parallel plate 164 from whichreflecting film 166 has been removed. Transparent adhesive 172 shouldgenerate the smallest possible optical loss since it is also applied tothe portion of parallel plate 164 forming a radiation window.

Referring now to FIG. 12 (D), a transparent protector plate 174 isapplied onto transparent adhesive 172 to protect reflecting film 166 andparallel plate 164. Since transparent adhesive 172 is applied to fillthe concave portion generated by removing reflecting film 166,transparent protector plate 174 can be provided in parallel with the topsurface of parallel plate 164.

Similarly, to protect reflecting film 168, an adhesive (not illustrated)can be applied to the top surface of reflecting film 168 and should beprovided with a protector plate (not illustrated). If reflecting film168 has a reflectance of about 100%, and there is no radiation window onthe same surface of parallel plate 164, then an adhesive and protectorplate do not necessarily have to be transparent.

Furthermore, an anti-reflection film 176 can be applied on transparentprotector plate 174. For example, transparent protector plate 174 andradiation window 170 can be covered with anti-reflection film 176.

According to the above embodiments of the present invention, a focalline is described as being on the surface of a radiation window or onthe opposite surface of a parallel plate from which input light enters.However, the focal line can be in the parallel plate, or before theradiation window.

According to the above embodiments of the present invention, tworeflecting films reflect light therebetween, with the reflectance of onereflecting film being approximately 100%. However, a similar effect canbe obtained with two reflecting films each having a reflectance of lessthan 100%. For example, both reflecting films can have a reflectance of95%. In this case, each reflecting film has light travellingtherethrough and causing interference. As a result, a luminous fluxtraveling in the direction depending on the wavelength is formed on bothsides of the parallel plate on which the reflecting films are formed.Thus, the various reflectances of the various embodiments of the presentinvention can easily be changed in accordance with requiredcharacteristics of a VIPA.

According to the above embodiments of the present invention, a waveguidedevice is described as being formed by a parallel plate, or by tworeflecting surfaces in parallel with each other. However, the plate orreflecting surfaces do not necessarily have to be parallel.

According to the above embodiments of the present invention, a VIPA usesmultiple-reflection and maintains a constant phase difference betweeninterfering lights. As a result, the characteristics of the VIPA arestable, thereby reducing optical characteristic changes causes bypolarization. By contrast, the optical characteristics of a conventionaldiffraction grating experience undesirable changes in dependance on thepolarization of the input light.

The above embodiments of the present invention are described asproviding luminous fluxes which are "spatially distinguishable" fromeach other. "Spatially distinguishable" refers to the luminous fluxesbeing distinguishable in space. For example, various luminous fluxes arespatially distinguishable if they are collimated and travel in differentdirections, or are focused in different locations. However, the presentinvention is not intended to be limited to these precise examples, andthere are many other ways in which luminous fluxes can be spatiallydistinguished from each other.

FIG. 13 is a diagram illustrating an apparatus which uses a VIPA as anangular dispersive component, instead of using diffraction gratings, toproduce chromatic dispersion, according to an embodiment of the presentinvention. Referring now to FIG. 13, a VIPA 240 has a first surface 242with a reflectivity of, for example, approximately 100%, and a secondsurface 244 with a reflectivity of, for example, approximately 98%. VIPA240 also includes a radiation window 247. However, VIPA 240 is notintended to be limited to this specific configuration. Instead, VIPA 240can have many different configurations as described herein.

As illustrated in FIG. 13, a light is output from a fiber 246,collimated by a collimating lens 248 and line-focused into VIPA 240through radiation window 247 by a cylindrical lens 250. VIPA 240 thenproduces a collimated light 251 which is focused by a focusing lens 252onto a mirror 254. Mirror 254 can be a mirror portion 256 formed on asubstrate 258.

Mirror 254 reflects the light back through focusing lens 252 into VIPA240. The light then undergoes multiple reflections in VIPA 240 and isoutput from radiation window 247. The light output from radiation window247 travels through cylindrical lens 250 and collimating lens 248 and isreceived by fiber 246.

Therefore, light is output from VIPA 240 and reflected by mirror 254back into VIPA 240. The light reflected by mirror 254 travels throughthe path which is exactly opposite in direction to the path throughwhich it originally travelled. As will be seen in more detail below,different wavelength components in the light are focused onto differentpositions on mirror 254, and are reflected back to VIPA 240. As aresult, different wavelength components travel different distances, tothereby produce chromatic dispersion.

FIG. 14 is a more detailed diagram illustrating the operation of theVIPA in FIG. 13, according to an embodiment of the present invention.Assume a light having various wavelength components is received by VIPA240. As illustrated in FIG. 14, VIPA 240 will cause the formation ofvirtual images 260 of beam waist 262, where each virtual image 260 emitslight.

As illustrated in FIG. 14, focusing lens 252 focuses the differentwavelength components in a collimated light from VIPA 240 at differentpoints on mirror 254. More specifically, a longer wavelength 264 focusesat point 272, a center wavelength 266 focuses at point 270, and ashorter wavelength 268 focuses at point 274. Then, longer wavelength 264returns to a virtual image 260 which is closer to beam waist 262, ascompared to center wavelength 266. Shorter wavelength 268 returns to avirtual image 260 which is farther from beam waist 262, as compared tocenter wavelength 266. Thus, the arrangement provides for normaldispersion.

Mirror 254 is designed to reflect only light in a specific interferenceorder, and light in any other interference order should be focused outof mirror 254. More specifically, as previously described, a VIPA willoutput a collimated light. This collimated light will travel in adirection in which the path from each virtual image has a difference ofmλ, where m is an integer. The mth order of interference is defined asan output light corresponding to m.

For example, FIG. 15 is a diagram illustrating various orders ofinterference of a VIPA. Referring now to FIG. 15, a VIPA, such as VIPA240, emits collimated lights 276, 278 and 280. Each collimated light276, 278 and 280 corresponds to a different interference order.Therefore, for example, collimated light 276 is collimated lightcorresponding to an (n+2)th interference order, collimated light 278 iscollimated light corresponding to an (n+1)th interference order, andcollimated light 280 is collimated light corresponding to an nthinterference order, wherein n is an integer. Collimated light 276 isillustrated as having several wavelength components 276a, 276b and 276c.Similarly, collimated light 278 is illustrated as having wavelengthcomponents 278a, 278b and 278c, and collimated light 280 is illustratedas having wavelength components 280a, 280b and 280c. Here, wavelengthcomponents 276a, 278a and 280a have the same wavelength. Wavelengthcomponents 276b, 278b and 280b have the same wavelength (but differentfrom the wavelength of wavelength components 276a, 278a and 280a).Wavelength components 276c, 278c and 280c have the same wavelength (butdifferent from the wavelength of wavelength components 276a, 278a and280a, and the wavelength of wavelength components 276b, 278b and 280b).Although FIG. 15 only illustrates collimated light for three differentinterference orders, collimated lights will be emitted for many otherinterference orders.

Since collimated lights at the same wavelength for differentinterference orders travel in different directions and are thereforefocused at different positions, mirror 254 can be made to reflect onlylight from a single interference order back into VIPA 240. For example,as illustrated in FIG. 15, the length of a reflecting portion of mirror254 should be made relatively small, so that only light corresponding toa single interference order is reflected. More specifically, in FIG. 15,only collimated light 278 is reflected by mirror 254. In this manner,collimated lights 276 and 278 are focused out of mirror 254.

A wavelength division multiplexed light usually includes many channels.Referring again to FIG. 13, if the thickness t between first and secondsurfaces 242 and 244 of VIPA 240 is set at a specific value, thearrangement will be able to simultaneously compensate for dispersion ineach channel.

More specifically, each channel has a center wavelength. These centerwavelengths are usually spaced apart by a constant frequency spacing.The thickness t of VIPA 240 between first and second surfaces 242 and244 should be set so that all of the wavelength components correspondingto the center wavelengths have the same output angle from VIPA 240 andthus the same focusing position on mirror 254. This is possible when thethickness t is set so that, for each channel, the round-trip opticallength through VIPA 240 travelled by the wavelength componentcorresponding to the center wavelength is a multiple of the centerwavelength of each channel. This amount of thickness t will hereafter bereferred to as the "WDM matching free spectral range thickness", or "WDMmatching FSR thickness".

Moreover, in this case, the round-trip optical length (2ntcosθ) throughVIPA 240 is equal to the wavelength corresponding to the centerwavelength in each channel multiplied by an integer for the same θ anddifferent integer, where n is the refractive index of the materialbetween first and second surfaces 242 and 244, θ indicates a propagationdirection of a luminous flux corresponding to the center wavelength ofeach channel. More specifically, as previously described, θ indicatesthe propagation direction of a resulting luminous flux as measured froma line perpendicular to the surface of reflecting surfaces 122 and 124.

Therefore, all of the wavelength components corresponding to the centerwavelengths will have the same output angle from VIPA 240 and thus thesame focusing position on mirror 254, if t is set so that, for thewavelength component corresponding to the center wavelength in eachchannel, 2ntcosθ is an integer multiple of the center wavelength of eachchannel for the same θ and different integer.

For example, a 2 mm physical length in round trip (which isapproximately double a 1 mm thickness of VIPA 240) and a refractiveindex of 1.5 enable all the wavelengths with a spacing of 100 GHz tosatisfy this condition. As a result, VIPA 240 can compensate fordispersion in all the channels of a wavelength division multiplexedlight at the same time.

Therefore, referring to FIG. 14, with the thickness t set to the WDMmatching FSR thickness, VIPA 240 and focusing lens 252 will cause (a)the wavelength component corresponding to the center wavelength of eachchannel to be focused at point 270 on mirror 254, (b) the wavelengthcomponent corresponding to the longer wavelength component of eachchannel to be focused at point 272 on mirror 254, and (c) the wavelengthcomponent corresponding to the shorter wavelength component of eachchannel to be focused at point 274 on mirror 254. Therefore, VIPA 240can be used to compensate for chromatic dispersion in all channels of awavelength division multiplexed light.

FIG. 16 is a graph illustrating the amount of dispersion of severalchannels of a wavelength division multiplexed light, in a case when thethickness t is set to the WDM matching FSR thickness, according to anembodiment of the present invention. As illustrated in FIG. 16, all thechannels are provided with the same dispersion. However, the dispersionsare not continuous between the channels. Moreover, the wavelength rangefor each channel at which VIPA 240 will compensate for dispersion can beset by appropriately setting the size of mirror 254.

If the thickness t is not set to the WDM matching FSR thickness,different channels of a wavelength division multiplexed light will befocused at different points on mirror 254. For example, if the thicknesst is one-half, one-third or some other fraction of the round tripoptical length thickness, then focusing points of two, three, four ormore channels may be focused on the same mirror, with each channel beingfocused at a different focusing point. More specifically, when thethickness t is one-half the WDM matching FSR thickness, the light fromodd channels will focus at the same points on mirror 254, and the lightfrom even channels will focus at the same points on mirror 254. However,the lights from the even channels will be focused at is different pointsfrom the odd channels.

For example, FIG. 17 is a diagram illustrating different channels beingfocused at different points on mirror 254. As illustrated in FIG. 17,wavelength components of the center wavelength of even channels arefocused at one point on mirror 254, and wavelength components of thecenter wavelength of odd channels are focused at a different point. As aresult, VIPA 240 can adequately compensate for dispersion in all thechannels of a wavelength division multiplexed light at the same time.

There are several different ways to vary the value of the dispersionadded by a VIPA. For example, FIG. 18 is a diagram illustrating a sideview of an apparatus which uses a VIPA to provide variable dispersion tolight, according to an embodiment of the present invention. Referringnow to FIG. 18, VIPA 240 causes each different interference order tohave a different angular dispersion. Therefore, the amount of dispersionadded to an optical signal can be varied by rotating or moving VIPA 240so that light corresponding to a different interference order is focusedon mirror 254 and reflected back into VIPA 240.

FIG. 19 is a diagram illustrating a side view of an apparatus which usesa VIPA to provide variable dispersion, according to an additionalembodiment of the present invention. Referring now to FIG. 19, therelative distance between focusing lens 252 and mirror 254 is maintainedconstant, and focusing lens 252 and mirror 254 are moved togetherrelative to VIPA 240. This movement of focusing lens 252 and mirror 254changes the shift of light returning to VIPA 240 from mirror 254, andthereby varies the dispersion.

FIGS. 20(A) and 20(B) are diagrams illustrating side views ofapparatuses which use a VIPA to provide various values of chromaticdispersion to light, according to additional embodiments of the presentinvention. FIGS. 20(A) and 20(B) are similar to FIG. 14, in that FIGS.20(A) and 20(B) illustrate the travel directions of a longer wavelength264, a center wavelength 266 and a shorter wavelength 268 of lightemitted by a virtual image 260 of beam waist 262.

Referring now to FIG. 20(A), mirror 254 is a convex mirror. With aconvex mirror, the beam shift is magnified. Therefore, a large chromaticdispersion can be obtained with a short lens focal length and a smallamount of space. When mirror 254 is convex, as in FIG. 20(A), the convexshape can typically only be seen from a side view and cannot be seenwhen viewed from the top.

Referring now to FIG. 20 (B), mirror 254 is a concave mirror. With aconcave mirror, the sign of the dispersion is inverted. Therefore,anomalous dispersion can be obtained with a short lens focal length anda small space. When mirror 254 is concave, as in FIG. 20 (B), theconcave shape can typically only be seen from a side view and cannot beseen when viewed from the top.

Mirror 254 can also be a concave or a convex mirror when viewed by thetop, thereby indicating that the mirror is a "one-dimensional" mirror.

In FIGS. 20 (A) and 20 (B), mirror 254 is located at or near the focalpoint of focusing lens 252.

FIG. 21 is a diagram illustrating a side view of an apparatus which usesa VIPA to provide variable dispersion to light, according to a furtherembodiment of the present invention. Referring now to FIG. 21, focusinglens 252 and mirror 254 are replaced with a retroreflector 282.Preferably, retroreflector 282 has two or three reflecting surfaces andreflects incident light in the opposite direction from the propagationdirection of the incident light. The use of retroreflector 282 willcause the VIPA-retroreflector arrangement to add anomalous dispersion.Moreover, retroreflector 202 is movable with respect to VIPA 240, tovary the amount of dispersion.

FIG. 22 is a diagram illustrating a top view of the apparatus in FIG.13, combined with a circulator, according to an embodiment of thepresent invention. Referring now to FIG. 22, a circulator 284 receivesinput light from an input fiber 286 and provides the input light tocollimating lens 248. Output light reflected by mirror 254 and backthrough VIPA 240 is received by circulator 284 and provided to an outputfiber 288. In FIG. 22, focusing lens 252 is a "normal" focusing lens,where a "normal" focusing lens refers to a focusing lens which focuseslight as seen from both a top view and a side view of the focusing lens.

FIG. 23 is a diagram illustrating a top view of an apparatus which usesa VIPA to add dispersion to light, according to an additional embodimentof the present invention. Referring now to FIG. 23, a cylindrical lens290 line-focuses light output from VIPA 240 to mirror 254. Mirror 254 isslightly tilted when viewed from is the top (as in FIG. 23). An inputfiber 292 provides input light to collimating lens 248, and an outputfiber 294 receives light reflected by mirror 254 and back through VIPA240. Therefore, by using cylindrical lens 290 and tilting mirror 254, itis not necessary to use a circulator (such as circulator 284 illustratedin FIG. 22).

A VIPA, according to the above embodiments of the present invention,provides a much larger angular dispersion than a diffraction grating.Therefore, a VIPA as described herein can be used to compensate for muchlarger chromatic dispersion than a spatial grating pair arrangement asillustrated in FIGS. 6(A) and 6(B).

In the above embodiments of the present invention, a mirror is used toreflect light back into VIPA 240. Thus, a mirror can be referred to as a"light returning device" which returns light back to VIPA 240. However,the present invention is not intended to be limited to the use of amirror as a light returning device. For example, a prism (instead of amirror) can be used as a light returning device to return light back toVIPA 240. Moreover, various combinations of mirrors and/or prisms, orlens apparatuses can be used as a light returning device to return lightback to VIPA.

In the above embodiments of the present invention, a VIPA has reflectingfilms to reflect light. For example, FIG. 8 illustrates a VIPA 76 havingreflecting films 122 and 124 to reflect light. However, it is notintended for a VIPA to be limited to the use of "film" to provide areflecting surface. Instead, the VIPA must simply have appropriatereflecting surfaces, and these reflecting surfaces may or may not beformed by "film".

Further, in the above embodiments of the present invention, a VIPAincludes a transparent glass plate in which multiple reflection occurs.For example, FIG. 8 illustrates a VIPA 76 having a transparent glassplate 120 with reflecting surfaces thereon. However, it is not intendedfor a VIPA to be limited to the use of a glass material, or any type of"plate", to separate the reflecting surfaces. Instead, the reflectingsurfaces must simply be maintained to be separated from each other bysome type of spacer. For example, the reflecting surfaces of a VIPA canbe separated by "air", without having a glass plate therebetween.Therefore, the reflecting surfaces can be described as being separatedby a transparent material which is, for example, optical glass or air.

As described above, the operation of a VIPA is sensitive to thethickness and the refractive index of the material between thereflecting surfaces of the VIPA. In addition, the operational wavelengthof a VIPA can be precisely adjusted by controlling the temperature ofthe VIPA.

More specifically, FIG. 24 is a diagram illustrating a controller forcontrolling the temperature of a VIPA, according to an embodiment of thepresent invention. Referring now to FIG. 24, a VIPA 300 produces anoutput light 302. A temperature sensor 304 detects the temperature ofVIPA 300. Based on the detected temperature, a controller 306 controls aheating/cooling element 308 to control the temperature of VIPA 300 toadjust the operational wavelength of VIPA 300.

For example, raising and lowering the temperature of VIPA can slightlychange the output angle of output light 302. Moreover, an output lightcorresponding to a specific wavelength of input light should be outputfrom VIPA 300 at a precise output angle. Therefore, controller 306adjusts the temperature of VIPA 300 so that output light 302 is properlyoutput at the correct output angle, and remains stable.

According to the above embodiments of the present invention, anapparatus uses a VIPA to compensate for chromatic dispersion. For thispurpose, the embodiments of the present invention are not intended to belimited to a specific VIPA configuration. Instead, any of the differentVIPA configurations discussed herein, or those disclosed in related U.S.application Ser. No. 08/685,362, which is incorporated herein byreference, can be used in an apparatus to compensate for chromaticdispersion. For example, the VIPA may or may not have a radiationwindow, and the reflectances on the various surfaces of the VIPA are notintended to be limited to any specific examples.

Although a few preferred embodiments of the present invention have beenshown and described, it would be appreciated by those skilled in the artthat changes may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

What is claimed is:
 1. An apparatus receiving an input light focusedinto a line and producing a spatially distinguishable output light, theapparatus comprising:first and second surfaces separated from each otherwith air in between, the second surface having a reflectivity whichallows a portion of light incident thereon to be transmittedtherethrough, the first and second surfaces being positioned so that theinput light travels from the line to be reflected a plurality of timesbetween the first and second surfaces through the air to cause aplurality of lights to be transmitted through the second surface, theplurality of transmitted lights interfering with each other to producethe output light, wherein the input light is at a respective wavelengthwithin a continuous range of wavelengths and the output light isspatially distinguishable from an output light formed for an input lighthaving any other wavelength within the continuous range of wavelengths.2. An apparatus as in claim 1, wherein the first and second surfaces areparallel with each other.
 3. An apparatus as in claim 1, wherein thereflectance of the first surface is substantially 100%.
 4. An apparatusas in claim 1, wherein the reflectance of the second surface is greaterthan 80% and less than 100%.
 5. An apparatus as in claim 1, wherein theinput light comprises at least two lights which each are at a differentwavelength, and the plurality of transmitted lights interfere with eachother to produce a respective output light for each light of the inputlight, each output light being spatially distinguishable from the otheroutput lights.
 6. An apparatus as in claim 1, wherein the input light isa wavelength division multiplexed light comprising at least two carrierswhich each are at a different wavelength, and the plurality oftransmitted lights interfere with each other to produce a respectiveoutput light for each carrier of the input light, each output lightbeing spatially distinguishable from the other output lights.
 7. Anapparatus as in claim 6, wherein each output light propagates in adifferent direction than each of the other output lights, to thereby bespatially distinguishable.
 8. An apparatus receiving a wavelengthdivision multiplexed light focused into a line and comprising at leasttwo carriers, the apparatus producing a spatially distinguishable outputlight for each carrier, the apparatus comprising:first and secondsurfaces separated from each other with air in between, the secondsurface having a reflectivity which allows a portion of light incidentthereon to be transmitted therethrough, the first and second surfacespositioned so that the wavelength division multiplexed light travelsfrom the line to be reflected a plurality of times between the first andsecond surfaces through the air to cause a plurality of lights to betransmitted through the second surface, the plurality of transmittedlights interfering with each other to produce a respective output lightfor each carrier of the wavelength division multiplexed light, whereineach carrier is at a respective wavelength within a continuous range ofwavelengths and the output light formed for a respective carrier isspatially distinguishable from an output light formed for a carrierhaving any other wavelength within the continuous range of wavelengths.9. An apparatus as in claim 8, wherein the first and second surfaces areparallel with each other.
 10. An apparatus as in claim 8, wherein thereflectance of the first surface is substantially 100%.
 11. An apparatusas in claim 8, wherein the reflectance of the second surface is greaterthan 80% and less than 100%.
 12. An apparatus comprising:first andsecond surfaces separated from each other with air in between, thesecond surface having a reflectivity which causes a portion of lightincident thereon to be transmitted therethrough, whereinan input lightat a respective wavelength is focused into a line, and the first andsecond surfaces are positioned so that the input light radiates from theline to be reflected a plurality of times between the first and secondsurfaces through the air and thereby cause a plurality of lights to betransmitted through the second surface, the plurality of transmittedlights interfering with each other to produce an output light which isspatially distinguishable from an output light produced for an inputlight at a different wavelength.
 13. An apparatus as in claim 12,wherein the input light comprises at least two lights which each are ata different wavelength, and the apparatus produces a respective outputlight for each light of the input light, each output light beingspatially distinguishable from the other output lights.
 14. An apparatusas in claim 13, wherein each output light propagates in a differentdirection than each of the other output lights, to thereby be spatiallydistinguishable.
 15. An apparatus as in claim 12, wherein the first andsecond surfaces are parallel with each other.
 16. An apparatus as inclaim 12, wherein the reflectance of the first surface is substantially100%.
 17. An apparatus as in claim 12, wherein the reflectance of thesecond surface is greater than 80% and less than 100%.
 18. An apparatuscomprising:first and second surfaces separated from each other with airin between, the second surface having a reflectivity which causes aportion of light incident thereon to be transmitted therethrough,whereinan input light at a respective wavelength is focused into a line,and the first and second surfaces are positioned so that the input lightradiates from the line to be reflected a plurality of times between thefirst and second surfaces through the air and thereby cause a pluralityof lights to be transmitted through the second surface, each transmittedlight interfering with each of the other transmitted lights to producean output light which is spatially distinguishable from an output lightproduced for an input light at a different wavelength.
 19. An apparatusas in claim 18, wherein the input light comprises at least two lightswhich each are at a different wavelength, and the apparatus produces arespective output light for each light of the input light, each outputlight being spatially distinguishable from the other output lights. 20.An apparatus as in claim 19, wherein each output light propagates in adifferent direction than each of the other output lights, to thereby bespatially distinguishable.
 21. An apparatus receiving a line focusedinput light having a respective wavelength within a continuous range ofwavelengths, the apparatus comprising:first and second surfaces spacedapart from each other with air in between; and means for causingmultiple reflection of the line focused input light between the firstand second surfaces through the air to produce self-interference thatforms an output light, wherein the output light is spatiallydistinguishable from an output light formed for an input light havingany other wavelength within the continuous range of wavelengths.
 22. Anapparatus receiving an input light at a respective wavelength andfocused into a line, the apparatus comprising:first and second surfacesspaced apart from each other with air in between; and means for causingthe input light to radiate from the line to be reflected a plurality oftimes between the first and second surfaces through the air and therebycause a plurality of lights to be transmitted through the secondsurface, and for causing the transmitted lights to interfere with eachother to produce an output light which is spatially distinguishable froman output light produced for an input light at a different wavelength.